1. Field of the Invention
This invention relates to a photovoltaic relay, and in particular to a photovoltaic AC/DC analog switch which includes a unique pull down circuit for rapidly discharging the gate-to-source capacitance (including all parasitic and intrinsic capacitance) of the relay's field effect transistor(s).
2. PRIOR ART
Solid-state relays are rapidly replacing reed relays in many applications. Photovoltaic AC/DC analog switches provide electric isolation comparable to reed relays and also provide superior speed and reliability without "bounce". However, while such switches are faster than reed relays, speed has remained a major problem. In particular, prior art photovoltaic AC/DC switches turn off at a lower speed than desired.
FIG. 1 shows a diagram of a prior art photovoltaic AC/DC analog switch 18, which includes photovoltaic stack S of 10 series-connected photodiodes 1 through 10 connected between the gate and source of N channel field effect transistor (FET) F1. Switch 18 is a unidirectional switch suitable for passing or blocking an electrical current from terminal T1 to terminal T2. When it is desired to turn on switch 18, a light source 17, typically a light-emitting diode (LED), is turned on, and light from light source 17 impinges on the photodiode stack S to generate a photocurrent in each photodiode 1 through 10. This photocurrent flows into the gate G1 of field effect transistor F1, charging the gate-to-source capacitance and causing a gate-to-source voltage drop across F1. However, since the photocurrent generated by each diode is low, typically 1 to 5 microamps, and since the gate capacitance of F1 is large, typically 100 to 400 picofarads, the time required to charge the gate-to-source capacitance is long, approximately 200 to 2000 microseconds. The maximum total voltage across the photodiode stack is the sum of the open circuit voltages of photodiodes 1 through 10. The voltage on the gate of F1 rises asymptotically to this maximum level, which is generally selected to be substantially above the threshold voltage of field effect transistor F1. As a result of the gate-to-source voltage drop across F1, F1 turns on to form a low impedance path from T1 to T2.
When it is desired to turn switch 18 off, light source 17 is turned off, turning off photodiodes 1 through 10. The gate-to-source capacitance of F1 begins to discharge. However, in the absence of pull down circuit 11 connected between node A and node B, the discharge current is only the leakage current of diode stack S, approximately 10 to 1000 picoamps. This makes the turn off time of field effect transistor F1 extremely long, typically 1 to 1000 milliseconds. In order to reduce this lengthy turn off time, various prior art pull down circuits 11 have been connected between nodes A and B.
FIGS. 2a through 2d illustrate various prior art pull down circuits for use with the circuit of FIG. 1. These circuits have been produced by Dionics Inc. of Westbury, N.Y. FIG. 2a shows a pull down circuit 11 consisting of resistor R connected between nodes A and B. By decreasing the resistance of resistor R, the turnoff time of field effect transistor F1 is decreased. Typically, the value of resistor R is selected to be between 1 and 20 megaohms. The circuit of FIG. 2a has the disadvantage that the smaller the value of resistor R, the more current is shunted away from the gate of field effect transistor F1 during turn on, which increases the turn on time.
FIG. 2b shows a pull down circuit 11 including phototransistor 12 connected between nodes A and B which is driven by a separate LED 19. When LED 17 is on, LED 19 is held off by external circuitry (not shown) and thus phototransistor 12 is off. This allows the photocurrent from stack S to charge the gate-to-source capacitance of FET F1, turning F1 on. When LED 17 is turned off, LED 19 is turned on by external circuitry (not shown). Light from LED 19 then turns on phototransistor 12 which discharges photodiode stack S as well as the gate-to-source capacitance of FET F1. FIG. 2c shows a pull down circuit 11 which includes a separate photodiode stack 13 which is driven by LED 14. When LED 17 is turned on, LED 14 is held off by external circuitry (not shown) and thus FET F1 is turned on as explained above. When LED 17 is turned off, LED 14 is turned on (by external circuitry not shown) which causes photodiode stack 13 to draw current and discharge photodiode stack S and the gate-to-source capacitance of FET F1.
FIG. 2d shows a pull down circuit composed of a depletion mode FET 15 with an additional photodiode stack 16 connected between the gate and the source of FET 15. Photodiode stack 16, which may have between 1 and 20 photodiodes, is driven by the same LED 17 as photodiode stack S. When LED 17 is turned on, both stack S and stack 16 are turned on. Stack S charges the gate-to-source capacitance of FET F1, turning on FET F1. Simultaneously, stack 16 charges the gate-to-source capacitance of depletion mode FET 15 which turns off FET 15. When LED 17 is turned off, the voltage across stack 16 drops rapidly since depletion mode FET 15 has a very low gate capacitance. Thus, depletion mode FET 15 turns on, discharging stack S and the gate-to-source capacitance of FET F1.
While the above pull down circuits improve the turn off time of field effect transistor F1, the best turn off time achieved by circuits 2a through 2d is approximately 400 to 800 microseconds, which is still relatively slow. The circuits shown in FIGS. 2b and 2c have the disadvantage of requiring two separate LEDs which are on alternately It is thus very impractical to implement these circuits in a single integrated circuit since stray light from one LED may trigger the device driven by the other LED.
A pull down circuit requiring a separate photodiode stack, e.g., the circuit of FIG. 2c, has the additional disadvantage of a large area penalty in an integrated circuit embodiment.
Therefore, one of the objects of this invention is to provide a photovoltaic relay which includes a pull down circuit for discharging the gate-to-source capacitance of field effect transistor(s) in a photovoltaic relay more rapidly than prior art circuits, without degrading the turn on time of the field effect transistor(s). A further object of the invention is to provide a photovoltaic relay which uses only a single LED and can be implemented as a single silicon integrated circuit.